Vehicles emit highly volatile organic compounds. The emissions concern, on the one hand, so-called “non-fuel emissions” which belong in the field of material emissions, e.g. of plastic material parts, fresh paints, adhesives or sealing materials. On the other hand, the emissions concern so-called “fuel emissions” which occur in the field of fuel-conducting elements and components (fuel tank unit, fuel lines, internal combustion engine with attachments). Fuel emissions move essentially, when the engine is switched off, out of the engine interior (piston chamber) via open valves into the air intake tract and the exhaust gas unit. Emissions via the exhaust gas unit can be regarded as non-critical since the fuel vapours are retained/broken down via the catalytic converter. The order of magnitude of the fuel emissions from the engine interior via the air intake tract to the external atmosphere is, according to the type of engine, between 1 mg up to several grams over a period of time of time of 24 hours. Legally prescribed limits for the emissions of highly volatile compounds of entire vehicles are, at e.g. 0.5 grams within 24 hours (CARB—California Air Resources Board, EPA—Environmental Protection Agency), significantly lower. For this reason, the fuel emissions in the region of the engine must be significantly reduced via HC sinks (barriers which prevent the vapourisation of fuel) in order to be able to maintain the limits for the entire vehicle.
The reduction in emissions of highly volatile compounds in the motor vehicle field is achieved by extensive measures which extend over the entire vehicle. The measures comprise for example the use of new or modified materials in the non-fuel and in the fuel field but also the application of specific emission-reducing measures. In the air intake tract, new vehicle models are equipped at present, in addition to the air filter, with activated carbon fleeces in the air intake path which are intended to prevent escape of highly volatile organic compounds by adsorption in the form of a diffusion barrier. These activated carbon fleeces are cost-intensive, produce a dynamic pressure in the intake air (=>power losses, more consumption of fuel) during the driving operation and are limited in their dimensioning since they must at most be only a few millimetres thick in order not to lead to too great a power reduction. However, this in turn has a negative effect on the adsorption behaviour of the activated carbon fleeces. In the worst case, high concentrations lead to a so-called breakthrough. This means that the organic compounds move out of the motor interior to the external atmosphere despite the fleece, after saturation thereof, via diffusion processes and a vehicle does not observe the limit in an official test. In addition to the activated carbon fleeces, further passively acting systems, e.g. “HC catalytic converter”, “activated carbon bypass”, activated carbon covering of the wall materials of the air intake system etc., are at present being tested for their everyday serviceability, which systems exploit the diffusion behaviour of highly volatile organic compounds from the engine interior into the open air.
The US and particularly Californian legislation of the CARB authorities has assumed a role of front runner with the worldwide lowest limits in the field of total emissions of highly volatile organic compounds. The limits of the CARB legislation have been/are already taken up by other US States and nations (Korea, Japan) and are also binding there. The EU will correspondingly follow suit within the scope of new draft bills. Hence there is a great requirement to equip internal combustion engines such that a reduced emission of highly volatile organic compounds is ensured.